1. Field of the Invention
The present invention relates to a spectrally sensitized silver halide photographic emulsion and, more particularly, to a silver halide photographic emulsion supersensitized with a novel combination of two types of sensitizing dyes.
2. Description of the Prior Art
It is well known that the light-sensitive wavelength region of a silver halide photographic emulsion can be expanded and spectrally sensitized by adding a sensitizing dye to a silver halide photographic emulsion. In some cases, only one sensitizing dye is used to sensitize the emulsion to the desired spectral sensitization wavelengths. However, in many cases two or more sensitizing dyes are used in combination. When two or more sensitizing dyes are used in combination, the degree of spectral sensitization often is intermediate the degree achieved in using the two dyes alone or less than that achieved using the two dyes alone. In some cases, however, a specific combination of different types of sensitizing dyes markedly increases the degree of spectral sensitization as compared with the independent use of the sensitizing dyes. This phenomenon is usually called supersensitization by sensitizing dyes.
When such a combination is used, a shift of the spectral sensitization to a wavelength region which cannot be expected from the spectral sensitization of the independent use of the dyes will sometimes result, although the spectral sensitization wavelength usually is intermediate that of the two dyes used alone or is a mere combination of that of the two dyes used alone.
Discovery of the combination of sensitizing dyes which, when used in combination, can provide a higher degree of spectral sensitization as compared with their independent use and can exhibit a sensitization in a wavelength region suitable for the end-use of a photographic light-sensitive material has become an important subject in the art of spectrally sensitizing silver halide photographic emulsions.
In the combination of sensitizing dyes used for supersensitization, a great selectivity is required between the respective dyes, and an apparently small difference in the chemical structure greatly influences supersensitization. That is, which combination of sensitizing dyes will provide supersensitization is difficult to predict from only the chemical structures of the dyes employed.
The sensitizing dyes to be used in supersensitizing a silver halide photographic emulsion must not exert any detrimental action on the photographic additives other than the sensitizing dyes and should possess stable photographic properties upon storage of the light-sensitive materials.
Further, remaining color resulting from the sensitizing dyes to be used must not be left in a light-sensitive material after processing. In particular, in short time processing such as rapid processing (usually 3 seconds to 90 seconds), the dyes must not leave any remaining color. Particularly, this remaining color is undesirable with photographic papers. Because, in black-and-white photographs, a bright white cannot be obtained and, with color photographs, reproduction of a true color becomes impossible.
It is known to use sensitizing dyes in order to increase the sensitivity in the blue region, e.g., as disclosed in U.S. Pats. No. 3,752,670, 3,480,434, German Pat. Application (OLS) No. 2,303,204, Japanese Pat. Publication No. 30023/71, etc. However, the above-described sensitizing dyes used for the sensitization in the blue region do not possess sufficient photographic properties. That is, the simple merocyanine dyes described in U.S. Pat. No. 3,480,434, Japanese Pat. Publication No. 30023/71, etc., have the defect that they broaden the spectral sensitivity distribution of a silver halide emulsion to such a long wavelength side that even the green region is sensitized. This gives rise to a deterioration in the color reproducibility in multi-layered color light-sensitive materials. When a yellow filter layer is provided on a blue-sensitive layer in order to remove this defect, the blue light sensitivity is also reduced, although the sensitivity of the green region is reduced to some extent, and thus such is not preferred.
Also, of the monomethine cyanine dyes described in U.S. Pat. No. 3,752,670, German Patent Application (OLS) No. 2,303,204, etc., some dyes have the defect that the residual color is high, although they show a sharp J-band sensitization.
Therefore, an improved photographic emulsion in which the spectrally sensitized region in the blue region is sharply cut on the longer wavelength side and which gives rise to less dye stain in the processed photographic element is required.